1. Field of the Invention
The present invention relates to a method for manufacturing a siamese-type cylinder block and more particulary, to a method for manufacturing such a cylinder block in which a sleeve made of cst iron is incorporated in each cylinder barrel of a siamese-type cylinder barrel made of an aluminum alloy and consisting of a plurality of cylinder barrels connected in series.
The invention also relates to apparatus for casting a blank for such cylinder block.
2. Description of the Prior Art
In the prior art, a siamese-type cylinder block has been made by placing each sleeve in a siamese-type cylinder barrel molding cavity in a mold to cast a cylinder block blank in a die cast process and then, subjecting the inner peripheral surface of each sleeve to a working operation to form it into a true circle.
In this conventional process, however, each sleeve becomes deformed and has a substantially oval configuration in a section with the lengthwise axis perpendicular to the axis of the cylinder barrels because the opposed peripheral walls of the adjacent sleeves are intensely subjected to the pressure of the molten metal during introduction thereof.
In this case, the configuration in section of each cylinder barrel after shrinkage thereof upon solidication of the molten aluminum alloy is substantially oval with the lengthwise axis parallel to the direction of the aligned cylinder barrels and hence, each sleeve is subjected to the shrinkage force of the aluminum alloy and is deformed to follow the configuration in section of each cylinder barrel upon its shrinkage, and the deformed sleeve is changed in configuration at the introduction of the moltern metal to a slight extent.
This results in an oval configuration in section of each sleeve and barrel with their lengthwise axes offset approximately 90.degree. from each other. Hence, the casting stress remaining in each sleeve is nonuniform around its inner peripheral surface. When the sleeve in such a state is subjected to machining to form its inner peripheral surface into a true circle for assembly in an engine, this operation causes the sleeve to be thermally expanded nonuniformly around its circumference. For this reason, a clearance may be produced between a piston ring and the sleeve, resulting in an increased amount of blow-by gas and a useless consumption of oil.
In addition, in the conventional process, the sleeve has been cast in each cylinder barrel. At the outer peripheral surface of each sleeve, annular or spiral slip-off preventing grooves are formed at a predetermined pitch during the casting of the sleeve in the mold to extend in the circumferential direction over a predetermined length from the end of the sleeve at which a cylinder head is secured. The slip-off preventing grooves are generally U-shaped in cross section.
However, the use of the sleeve as cast prevents close adhesion because of microporosity of the outer peripheral surface of such sleeve and thus, a very small clearance may be produced between the sleeve and the cylinder barrel. If the slip-off preventing grooves is of U-shaped in cross section, then a gas, such as air can enter at the corners between the inner side and bottom surface of the groove during casting and be confined therein by the molten metal. This also causes a very small clearance to be produced between the sleeve and the cylinder barrel as described above. In a siamese-type cylinder block, the adjacent sleeves are very close to each other, and between these sleeves there is generally no water-jacket. Therefore, the heat at the portions of both the sleeves opposed to each other may be transferred in the shortest path to a water jacket through the barrel located between these sleeves. However, if a very small clearance, as described above, is produced around the outer periphery at those portions of both opposed sleeves, such heat transfer path is disconnected so that release of heat from the sleeve is not effected uniformly around its circumference. Thus, the efficiency in release of heat from the sleeve is reduced.
The shaping of individual slip-off preventing grooves by the mold results in a wide variation in depth of the grooves and in an unevenness in thickness of the sleeve at the slip-off preventing grooves and the land portions between the adjacent grooves.
In such a cylinder block, the amount of sleeve expansion is nonuniform around the circumference of the sleeve and hence, the same problems may arise as described above.